1. Field of the Invention
The present invention concerns radiographic imaging and in particular concerns the use of an optical camera system to recover the radiographic geometry of a radiographic system, and the use of the recovered radiographic geometry to generate and manipulate three-dimensional radiographic information from radiographic images.
2. Description of the Related Art
Radiographic imaging provides a valuable diagnostic tool with both medical and industrial applications. From standard two-dimensional radiographic images, three-dimensional radiographic information can be generated. For example, stereoscopic x-ray imaging techniques process a pair of radiographic images taken from different positions relative to a subject, and generate images of the subject that appear to be three-dimensional when viewed with the proper equipment. As another example, tomosynthesis synthesizes images of internal slices of a subject at specific depths into the subject by combining multiple radiographic images of the subject taken from different positions relative to the subject.
To generate and manipulate three-dimensional radiographic information from multiple radiographic images, the radiographic geometry must be known. In particular, it is necessary to know the exact geometrical relationship between the x-ray source, the subject of the x-ray, and the x-ray sensor. The radiographic geometry provides information used for processing and generating three-dimensional radiographic information such as correcting distortions in radiographic images for stereoscopic x-ray imaging, combining radiographic images when forming tomographic slices, and performing volume reconstruction.
In conventional systems, the radiographic geometry of the system is obtained through precise measurements and calibration of the x-ray source, the subject and the x-ray sensor. The requirement that these measurements be precise throughout the process of obtaining the multiple radiographic images often prevents the use of these systems in situations where it is difficult to obtain or maintain the geometry. Additionally, slight variations in the obtained measurements or the calibration process can lead to inaccuracies in the generated three-dimensional radiographic information.
Methods have been developed to help overcome the difficulties present in obtaining and maintaining the radiographic geometry of a system during the process of obtaining the radiographic images. One conventional method employed when unstable geometry is involved is optical tracking. With optical tracking, the system utilizes multiple visible light cameras configured in different positions relative to the subject together with markers attached to the portions of the subject that are unstable. The multiple cameras track movement of the subject by detecting the position of the markers. However, these systems have the disadvantage of requiring multiple visible light cameras configured in multiple directions relative to the subject as well as requiring the use of markers attached to the subject.
Self-calibrating systems for determining the radiographic geometry have also been developed for use in tomosynthesis. These systems typically involve positioning a radiopaque fiducial in a fixed position relative to the subject. Using the position of the fiducial in the radiographic images, the radiographic geometry of the system can be determined. The system then produces tomographic images of the subject using the calculated geometry. These types of systems also have disadvantages in that the use of radiopaque fiducials is required and the radiopaque fiducials can obscure portions of the radiographic image. Additionally, detecting the fiducials in the resulting radiographic images only provides relative geometry. Actual measurements are still required in order to determine the exact geometry of the system.